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Thread: Koi Pond Concrete Slab on Clay Soil

  1. #1
    Nightstorm's Avatar
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    Koi Pond Concrete Slab on Clay Soil

    Here's the short story. I've excavated my pond and dug too deeply on one side (the slope of the yard tricked me when excavating). I've shoveled a bunch of dirt back in, compacted it all with a jumping jack compactor, dug shallow trenches around the outside (tossing this soil into the center part), then tamped it down again. I'm still a few inches (in some places, could be 5") low.

    I'll be pouring a 6" slab with #4 rebar on 10" centers. The trenches will make the footers under the walls 10-12" with its own rebar.

    Here's my problem - or maybe its not a problem. Parts of the ground are soft. There are a couple of areas where the jumping jack compactor wanted to dig down into the soil. In the picture, you can see some tracks in the clay that were left behind. In some places with the compactor, it felt like I was on plywood that had some give.

    Is this a problem? Can I just pour the concrete slab over this and rely on the rebar and thickness of the slab, as well as the distributed nature of the load (nothing compared to cars on a driveway, for instance)? From what I've read, gravel is typically placed under a patio or driveway slab to handle ground water, and to reduce frost heaving. This thing is 3' below my frost line, so that's not an issue. But I'm wondering if gravel is necessary to stabalize the soil before pouring?

    Having dug this thing out, and then shoveled a bunch back in, I'm not about to dig it out again to put in a sizeable gravel layer everywhere. But, as I see it, I have three options on how to deal with the soft clay and the variable gap that exists now between where the surface is and where it needs to be raised to pour a 6" slab. I could:
    1. shovel more dirt and rent the compactor again to raise the surface uniformly. Will still be soft clay
    2. Leave it as is, and just have thicker concrete in some places.
    3. Buy a bunch of gravel and use the gravel to even out the surface to the appropriate level. Some parts might not have much gravel, and others might have up to 5" in spots.


    I'm guessing that the gravel might help reduce the softness of the clay? There shouldn't be any water issues once this thing is built, as I'll have a french drain (possibly two - one near the surface where water flows and a deeper back up). It's really a question of how solid the dirt has to be before pouring concrete, whether soft clay (even if compacted) is suitable, and if gravel is required or helps.

    In this picture, the scale can throw you. The two long legs of the L-shaped pond are about 23' - 25' each. The short wall that is most obvious in the picture (upper left) is over 13'.
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    ShawnB is offline
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    I think you'd been better off leaving the pond dug out to vigin soil and then using gravel to bring it back up to the grade level you wanted. Less likely to see any settlement or sinking this way in my opinion. Don't forget water weighs about 8lbs a gallon.

    If that's not a option, I'd give it a day or 2 to let the soft spots dry out and use a plate compactor instead of the jumping jack and try to compact it as much as possible. Then put down a layer of gravel to desired grade.

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    wbrad90 is offline
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    Get some recycled crush & run (fairly inexpensive) and use as fill. Based on your statement that you need to back fill no more than 5 additional inches, I would add approx. 3", Compact, level off and compact. Level again if needed and compact.

    How much soil did you add (depth) before you ran the compacter the first time? You may need to redo this if to much depth between compactions. I am not sure of the lift requirements for clay soil, so hopefully someone can chime in with that information.
    Walt


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    Chris

    I agree with ShawnB here. Let the soil dry out first and see how firm it gets. Then make a decision about adding the gravel or not.

    Mike
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    Hmmm, that might be tough to do. We're supposed to get rain for the next week. Although, the footer trenches around the outside might keep the middle part reasonably dry...

    On the plus side, I was talking to someone at work who used to be a civil engineer. His jaw nearly hit the floor when I told him it would be 6" of concrete with 1/2" rebar on 10" centers. He kept saying "You're building a structure with steel in it - that thing's not going anywhere"



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  6. #6
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    Clay will retain water for a long time, it will expand when dry and will crack your slab if not done propertly. To over come clay soil expansion in construction you have to beef up your rebar and slab thickness or go with great beams(bunch of footings all tie together under the slab) and use all 4,000 psi concrete.

    Make sure to have good drainage around the pond and the footings(burito drain is good and cheap)

    Tom
    Last edited by vipertom1970; 09-05-2012 at 12:38 AM.

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    Compacting soil is pretty complex. Clay can be too wet or too dry to compact. I assume the soil put back wasn't dripping wet like clay would be at the bottom of a flooded pit would be. If the clay was damp that would be about right although there are actual tests for these things. It sounds like maybe you compacted too much soil per pass. That can give to a layer of 1-2" of compacted soil over some amount of less compacted soil. That top layer spreads out the force so the lower soil won't compact. That will give you a bounce on top. Normally when too wet you'll see water appear at the surface as you compact.

    Either way the only fix is to dig it out and start over. Gravel can be mixed with too wet a clay for proper compaction. If that's not acceptable then it is what it is. Anything you add on top isn't going to fix problems beneath. Whether or not the concrete ends up cracking is impossible to tell. Certainly wouldn't be unexpected. A good inspector certainly wouldn't allow this.

    If this is going to be a liner pond cracking would be OK. The steel will keep it all basically together. If this is going to be a concrete pond with a paint on water proofing type deal I think you'd have to assume problems sometime down the line. You could always add a liner later. Just have to look at the amount of work for a proper fix vs expense of failure and or your goals.

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    I agree with Waterbug, it sounds like you compacted too thick a lift of material and would be best to dig that area that is bouncing back out and replace in 3" lifts compacting each one, there are liquid and plastic limit tests for clay and such materials that determine what needs done, (removing or adding moisture can make some clay useable).
    That makes things complicated, if you must dig it, using layeres of crush and run (limestone) would be great if not to much work or expensive, a possible simpler solution would be dig (if you have a tiller, makes this simple) back to solid/virgin as you place 3" of clay back in put 1/4 to 1/2" of portland cement on each layer rototill it in then compact (if damp your good to go, if dry mist before compacting) and continue this back to grade.
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    Chris,
    I believe that gravel being used mainly for drainage and prevent frost heaving as you have said. Gravel is not used under concrete slab to add strength or to increase load bearing capability, so in your case, replacing loose soil with gravel won't help you much. For me the most important thing is the footings. The footings must sit on virgin, undisturbed soil. Let's think about the parking garage or similar structures. There is nothing under the concrete slab, the whole concrete slab is supported by beams, posts and all load carrying on the footings. So if you have strong footings and the slab tied to the footings, what under the slab become less important. In my opinion, you can try to compact the soil as much as you can, but you don't need to dig out all of the soil and replace it with gravel. However, you must dig the footings to the virgin soil level. And because the size of the concrete slab is quite large, to be in the safe side, beside the footing around the perimeter, you may want to dig some additional trenches in the middle (to the virgin soil level) to add more footing across the middle of the slab to add more support similar to the support beams in the parking garage structure. It's just my opinion to give you some ideas, but I am not an engineer, so you should check with some one with more knowledge and experience in this area.
    Last edited by DCKOI; 09-05-2012 at 08:53 AM.

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